What you're missing is the residual pressure check valve....I don't know if you can put one in a disc/disc master cylinder....I've removed them from drum/drum master cylinders to make front discs work better....I suggest you get an A/G body master cylinder and hook it up just like the service manual for a Monte Carlo or Malibu tells you....Then you'll have a built-in residual pressure check valve that you need for your rear brakes....

I don't think GM has put the residual pressure valve in the master cylinder for a very long time (if ever). It will be in the combination valve (along with the proportioning valve, hold off valve, and idiot light switch).